The present application claims priority to Japanese Application No. P11-308153 filed Oct. 29, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a magnetic recording medium composed of a magnetic layer, main components of which are magnetic powder and a binder, and a non-magnetic substrate, on which the magnetic layer is formed, and further relates to a manufacturing method thereof.
2. Description of the Related Art
For manufacturing a coating type magnetic recording medium such as a magnetic tape and a magnetic disk, generally, a magnetic coating compound is first prepared by kneading ferromagnetic powder, a binder, a dispersant, a lubricant, an abrasive and the like in an organic solvent to disperse them. And then, the magnetic layer is formed by coating the magnetic coating compound on a non-magnetic substrate such as a polyester film, and further by drying the coated material.
The binder that is a main compounding ingredient together with the ferromagnetic powder in the ingredients constituting the magnetic coating compound is compounded for keeping the ferromagnetic powder being bound in the magnetic layer. Then such features are required for the binder as dispersibility of the ferromagnetic powder into the binder is good, and as the dispersibility of the ferromagnetic powder is not easily deteriorated when the magnetic coating compound is kept to be standing for a fixed term, and further as the dispersibility of the ferromagnetic powder does not deteriorate even if a lubricant or a crosslinking agent is added to the magnetic coating compound, namely dispersion stability of ferromagnetic power with the binder is good.
Conventionally, in such a kind of the field, there are used binders such as a vinyl chloride copolymer, a polyurethane resins an acrylonitrile-butadiene copolymer, a cellulosic resin, an epoxy resin, and an acrylic resin.
In this case, for example, a polyester polyurethane resin, to which a hydrophilic group is introduced in a moderate ratio, is frequently used for improving the absorption of the resin into the ferromagnetic powder or the dispersibility of the ferromagnetic powder. UR-8200 manufactured by Toyobo Co., Ltd. can be cited as a polyester polyurethane resin that has conventionally been used. The polyester polyurethane resin includes, in its polyester polyol part, ingredients such as a terephthalic acid or an isophthalic acid as a dicarboxylic acid ingredient and an ethylene glycol as a polyol (diol) ingredient.
On the other hand, recently, for realizing a change of the magnetic recording medium to be one having a high recording density, it is promoted to change the particles of the ferromagnetic powder to be dispersed in the magnetic layer to fine particles. However, solubility of the polyester polyurethane resin to an all-purpose organic solvent, including not only a separated solvent but also a mixed solvent, such as toluene, methyl ethyl ketone and acetic ether is not so good. Consequently, when the magnetic coating compound is prepared, the adsorptivity of a resin to the ferromagnetic powder and the dispersibility of the ferromagnetic powder become insufficient. Thereby, a stable resin adsorption layer was not obtained.
As a result, when the magnetic coating compound is preserved in a state of being standing or when a lubricant or a crosslinking agent is added to the magnetic coating compound while it is being stirred, a phenomenon that the dispersibility of the ferromagnetic powder is markedly deteriorated is observed. Consequently, a magnetic recording medium made by the use of the magnetic coating compound deficient in the dispersion stability could not give full scope to its satisfactory magnetostatic characteristic, electromagnetic transformation characteristic and surface characteristic of the medium.
Furthermore, for giving the strength necessary to the magnetic layer, various resins having high rigidity, into which a hydrophilic group is introduced, namely having a high glass transition temperature are developed. For example, MR-110 manufactured by Nippon Zeon Co., Ltd., which is one of vinyl chloride resins, as well as the aforesaid UR-8200, is a resin that belongs to the group. The glass transition temperature of them are both about 70xc2x0 C. As for the latter, especially, it has higher softness and abrasion resistance in comparison with those of the former having almost the same glass transition temperature because of the molecular structure and the molecular weight of the latter.
However, it is required for a recent magnetic recording medium to have strength exceeding the necessary amount thereof owing to the increment of the relative speed to a recording and reproducing head and the like. Consequently, it becomes impossible to cope with such a situation by the use of a resin having the aforesaid degree of the glass transition temperature.
The present invention was made for improving the aforesaid situation, and aims to provide a magnetic recording medium having improved running durability as well as an improved magnetostatic characteristic, an improved electromagnetic transformation characteristic and an improved running stability by means of a specified binder, and further aims to provide a manufacturing method thereof.
A magnetic recording medium according to an aspect of the present invention includes a magnetic layer, main components of which are magnetic powder and a binder, and a non-magnetic substrate on which the magnetic layer is formed, wherein at least a part of the binder is a polyurethane resin including a propanediol derivative expressed by the following general formula as an ingredient, and a urethane group concentration in the polyurethane resin is 4,000 or more equivalents per 1 t of the resin, 
where either or both of R1 and R2 in the general formula is a saturated hydrocarbon group having a carbon number of 2 to 8 and the remnant is the saturated hydrocarbon group or a hydrogen atom.
Furthermore, a manufacturing method of a magnetic recording medium according to an aspect of the present invention, in which, when magnetic powder and a binder are kneaded and dispersed to prepare a magnetic coating compound and a magnetic layer is formed on a non-magnetic substrate by means of the magnetic coating compound for manufacturing the magnetic recording medium, the magnetic coating compound is prepared by means of a polyurethane resin at least as a part of the binder. The polyurethane resin includes a propanediol derivative expressed by the aforesaid general formula as an ingredient, and a urethane group concentration in the polyurethane resin is 4,000 equivalents or more per lt of the resin.
The inventors of the present invention assiduously made their endeavor of research on the improvement of a binder to be used for a magnetic recording medium. As a result, the inventors found that the polyurethane resin including a propanediol derivative expressed by the aforesaid general formula as an ingredient and the specified urethane group concentration as mentioned above is a binder answering the purpose.
That is, the binder has good dispersibility of the ferromagnetic powder into it. Moreover, even if the magnetic coating compound is preserved in a standing state or a lubricant, a crosslinking agent and the like are added to the magnetic coating compound, the dispersibility of the ferromagnetic powder of the binder is not easily deteriorated and the glass transition temperature of the binder can be heightened, for example, to be 100xc2x0 C. or more.
Moreover, as it will be described, solubility of the polyurethane resin to the organic solvent often becomes worse as the urethane concentration becomes larger. In addition, since the ferromagnetic powder generally has a tendency to be easily coagulated as its particles are finer, it is difficult to disperse the ferromagnetic powder evenly in the magnetic layer. However, if the polyurethane resin defined as above is used as a binder, the dispersibility is improved and a magnetic coating compound having superior dispersibility and stability of the ferromagnetic powder is obtained. That is, it becomes possible to disperse the ferromagnetic powder evenly in the magnetic layer.
As a result, according to the present invention, there can be provided a magnetic recording medium having a good magnetostatic characteristic, a good electromagnetic transformation characteristic, a good running stability and good running durability.